1. Technical Field
The present disclosure relates to a dynamic bandwidth allocation method and an optical communication network, and particularly relates to bandwidth control on a passive optical network (PON).
2. Related Art
A telecommunications network that connects a building (central office) owned by a communications service provider to subscriber's premises is called an access network. With the recent increase in channel capacity, an optical access network that enables the transmission of enormous amounts of information by use of optical communication is going mainstream in the access network.
As a form of the optical access network, there is a passive optical network (PON). The PON includes one optical line terminal (OLT) provided in the central office, optical network units (ONUs) provided on a plurality of subscribers' premises respectively, and an optical splitter. The OLT, the ONUs and the optical splitter are connected with an optical fiber.
A single optical fiber is used for the connection between the OLT and the optical splitter. The single optical fiber is shared by a plurality of ONUs. Moreover, the optical splitter is an inexpensive passive element. In this manner, the PON is economical and easy to maintain, so that the PON is rapidly being introduced.
The PON uses various multiplexing technologies. Typical examples of the multiplexing technologies used by the PON include a time division multiplexing (TDM) technology, a wavelength division multiplexing (WDM) technology, and a code division multiplexing (CDM) technology. Time division multiplexing (TDM) is known as a system for allocating a short section on the time axis to each subscriber. Wavelength division multiplexing (WDM) is known as a system for allocating a different wavelength to each subscriber. Code division multiplexing (CDM) is known as a system for allocating a different code to each subscriber.
A TDM-PON using the TDM technology among these multiplexing technologies is widely being used at the moment. The PON using the multiplexing technology of TDM is disclosed in ““Technical Fundamentals Lecture: GE-PON Technology”, NTT Technical Journal, August 2005, pp. 71-74”, for example.
In the PON, a communication signal transmitted from the ONUs to the OLT (hereinafter sometimes referred to as the upstream signal) is multiplexed by the optical splitter and transmitted to the OLT.
On the other hand, a communication signal transmitted from the OLT to the ONUs (hereinafter sometimes referred to as the downstream signal) is demultiplexed by the optical splitter and transmitted to the ONUs.
Here, the upstream signal includes an upstream data signal and an upstream control signal. The upstream data signal is a signal transmitted from user equipment connected to the ONU to an upper network connected to the OLT.
In other words, the upstream data signal is a signal carrying information requested by a user.
On the other hand, the upstream control signal is a signal transmitted by a ONU controller included in the ONU to a optical line terminal controller included in the OLT, and is a signal used to control a network.
Moreover, the downstream signal includes a downstream data signal and a downstream control signal. The downstream data signal is a signal transmitted from the upper network to the user equipment.
In other words, the downstream data signal is a signal carrying information requested by the user.
On the other hand, the downstream control signal is a signal transmitted by the optical line terminal controller to the ONU controller, and is a signal used to control the network.
Time division multiple access (TDMA) is used for the TDM-PON.
TDMA is a technology where the OLT manages the transmission timing of each ONU and controls the upstream signals from different ONUs not to collide.
On the TDM-PON, the time axis is divided into a plurality of short sections, and the sections are allocated to the ONUs as communication time bandwidth for transmitting the upstream signals to the OLT.
The OLT instructs the ONUs on the transmission timing and the bandwidth of the upstream signal.
In this manner, that the OLT allocates communication time bandwidth to the ONUs is called bandwidth allocation.
Incidentally, the unit of “bandwidth” mentioned herein is given in time (seconds, for example).
Known as one of methods for allocating bandwidth is dynamic bandwidth allocation (DBA).
DBA is a bandwidth allocation method where the OLT receives transmission bandwidth requests from the ONUs and dynamically allocates communication bandwidth considering the bandwidth requests.
In DBA, a method for determining bandwidth to be allocated depending on queue lengths reported by the ONUs is especially called status reporting (SR)-DBA.
In SR-DBA, bandwidth to be allocated is updated at certain cycles (sometimes referred to as the allocation cycle).
In this case, the upstream control signal carries information on bandwidth that the ONU requests from the OLT, such as a queue length.
Moreover, the downstream control signal carries information on bandwidth that the OLT grants a transmission to the ONU, in other words, information on transmission timing and bandwidth.
SR-DBA known in related art can be classified into one handling the allocation cycle as a fixed parameter (hereinafter sometimes referred to as the fixed cycle mode) and one handling the allocation cycle as a variable parameter (hereinafter sometimes referred to as the floating cycle mode). For example, SR-DBA is disclosed in “Lam, Cedric. “Passive Optical Networks: Principles and Practice”. Elsevier Inc., 2007”.
In SR-DBA in the fixed cycle mode, firstly, the OLT allocates bandwidth corresponding to minimum guaranteed bandwidth (MinBW) to the ONUs.
The minimum guaranteed bandwidth is bandwidth that is previously decided under contract between a communications service provider and a user, and is guaranteed to be surely allocated even if communication traffic is congested. The minimum guaranteed bandwidth is a minimum value of bandwidth allocation.
Therefore, in SR-DBA in the fixed cycle mode, a fixed communication time decided based on the minimum guaranteed bandwidth is allocated as communication time bandwidth.
If there is still remaining bandwidth being unallocated space upon completion of allocating the minimum guaranteed bandwidth to all the ONUs, bandwidth allocation is performed on the remaining bandwidth as best-effort bandwidth.
The allocation of best-effort bandwidth is performed by distributing the remaining bandwidth at the rate of the minimum guaranteed bandwidth, for example.
Regardless of any amount of bandwidth requests of the ONUs, the allocation cycle is constant in SR-DBA in the fixed cycle mode.
In the following description, a constant allocation cycle may be referred to as the fixed allocation cycle in SR-DBA in the fixed cycle mode.
On the other hand, queue lengths notified by the ONUs are basically allocated as they are as bandwidth in SR-DBA in the floating cycle mode.
Considering the implementation of minimum guaranteed bandwidth services, the ONUs notify, as queue lengths, buffer amounts whose upper limits are set to the minimum guaranteed bandwidth.
Here, in SR-DBA in the floating cycle mode, the communication time necessary to transmit or receive is calculated based on the queue length notified by the ONU to set the commutation time as bandwidth.
The allocation cycle is determined by the sum total of bandwidth requested by the ONUs in SR-DBA in the floating cycle mode and accordingly varies depending on the number of ONUs that request bandwidth.